I must have missed something in your reply.. how exactly is a local shared memory cache on a mmap'd file, going to help me speed up external HEAD requests to dozens/hundreds of separate resources over HTTP?

For access to my templates and local files, sure, I can see how this would help (but so does Memoize, HTML::Template::Compiled, and so on), but I'm not sure where this helps speed up remote requests.

You mentioned caching the requests instead of actually doing them every time. This is a good way to do that. It's more efficient than a bunch of database queries, and makes it very easy to control timeouts on the cache, in case you only want to check every 2 hours or so.